Known within long-range surveillance and observation systems is the method of target detection and identification using TV systems that include a camera and a long focal lens.
The light collecting capability of long focal lenses is limited due to volume, weight and cost constraints.
Even when a camera includes in its setup an SLS (Star Light System), under conditions of inferior visibility, the natural background light intensity is not sufficient to enable the production of an image with adequate signal to noise ratio for exploiting the total resolution capability of the camera and to discern the fine details of the target in order to obtain its identification in the image.
At night such TV systems require adding an auxiliary light source illuminating the target in order to improve the received picture quality. Such an auxiliary light source can be a laser device capable of producing a light beam that is parallel to the line of sight (hereinafter LOS) of the camera and that illuminates the field of view (hereinafter FOV) of the camera or a part thereof.
A known problem inherent in surveillance and observation systems is the need to overcome inclement conditions such as: humidity, haze, fog, mist, smoke, or rain that might be present in the space between the surveillance and observation systems and the target being observed. A similar problem exists for observation systems operating in other media, for example, the influence of scattering in water in underwater observations being performed either from the air or in the water itself.
In TV surveillance and observation systems that are integrated with a laser device acting as the illuminating source as mentioned above, interference in the media between the system and the target, for example haze resulting from aerosols hovering in the air, in the case of atmospheric media, can cause backscatter of part of the laser beam. The backscatter of the laser beam results in self-blinding of the camera and thus reduces the contrast of the target relative to the background. Under nighttime conditions, contrast reduction results in significant lowering of the efficiency of target detection and identification in comparison to the attainable efficiency of target detection and identification in daytime light conditions.
In order to reduce the negative influences present in the space between the laser coupled TV surveillance and observation system and the target, the TV camera sensor is synchronized in time with the time in which the reflected energy from the laser illuminated target is due to be received in the optical assembly.
In this approach, a laser generates short light pulses at a given frequency with the TV camera activated at a similar frequency. The TV camera however is activated with a time delay that corresponds relatively to the frequency of the laser pulses.
Thus, when the laser beam light is sent to the target, the camera reception function is set to the OFF state. The laser light, traveling at the speed of light towards the target, impinges on the target and illuminates it and its nearby surroundings. A small part of the laser light is reflected back towards the camera.
Laser light reflected backwards as light reflexes from the media, for example the atmosphere, that is significantly close to the camera (relative to the distance between the camera and the target), reaches the camera when it is still set to the OFF state. The light is thus not received by the camera and does not influence nor reduce the contrast of the image.
In contra distinction, the light reflexes that reach the camera from the target and its adjacent surroundings arrive when the camera is already in the ON state, i.e. the reception state, and are thus fully collected.
The camera switches from the OFF to ON state in a time synchronized manner with the time it takes the pulse to travel to the target and back.
After reception of the image of the target, its adjacent vicinity, and its subsequent storage, the camera reverts to the OFF state and the system awaits the transmission of the next laser pulse.
This procedure is cyclically repeated in a rate established in accordance with the range to target, the speed of light, and the limitations set by the laser device and the camera.
Implementing this procedure enables the production of a dynamic image in real time.
The solution presented above is known as using gated television/TV to minimize backscatter by gating images of any intervening media between the target and the optical assembly.
U.S. Pat. No. 5,408,541 to Sewell entitled “Method and system for recognizing targets at long range ranges” describes a method that includes detection of the target, conducting a preliminary measurement of the range to target, and calculating the position relative to the coordinates in which the target was detected. Subsequently, the range data is fed into a gated television sensor that serves as the imaging device. Thereafter, the estimated area of the target is illuminated by a pulsed laser, in accordance with the measured range and relative location data. The energy returned from the target is processed and converted to display as a TV image.
U.S. Pat. No. 4,920,412 to Gerdt et al entitled “Atmospheric obscurant penetrating target observation system with range gating” describes a system for imaging a scene, obscured by atmospheric obscurants, and determining the range to illuminated targets in the scene. The system includes a television camera with a gated image intensifier. Short intense laser pulses are transmitted to different range slices in a scene in order to illuminate the scene. The image intensifier is gated on after a time delay equal to the round trip transit time of the pulse from a range slice of interest. The image intensifier is gated on for a time interval equal to the width of the laser pulse. One laser pulse per frame is transmitted and successive range slices are observed during successive frames by successively increasing the time delay. The range slice images are stored in a buffer and read out to a television display.
GB Patent No. 2,308,763 to Bagnall-Wild entitled “Laser range finders” describes a method and a system for reducing the reception of spurious reflected signals, termed ‘clutter,’ in laser range finders. The method includes selecting a pulse from a target object from a series of pulses including pulses reflected from clutter objects. Depending on the circumstance, either the last received pulse which exceeds a predetermined fraction of the maximum pulse amplitude is selected, or the first received pulse which exceeds a predetermined fraction of the maximum pulse amplitude is selected. The method also includes selecting a range window and discarding those pulses which lie outside the window, and defining a condition or set of conditions which enable the level of overspill of a laser light pulse over a target to be classified as either ‘high’ or ‘low.’
It is noted that Sewell requires a preliminary range measurement by a designated laser range finder (ranger). The measuring line to the target of the laser ranger has to be parallel, in a very accurate manner, to the LOS of the observation system. An instrument of this kind can be bulky (both large and heavy), relatively expensive, and not necessarily applicable to all types of surveillance and observation systems. It is noted that the televised image, of the system of Gerdt, is constantly rewritten during observation, and that the image appears similar to slow scan television and may be slightly erratic for fast moving ships.
Thus there is a need for an observation system for day and night applications, based on the gated imaging principle, which can adapt to long range observations, which does not necessitate a preliminary range to target measurement.